Organic EL devices are known to be highly efficient and are capable of producing a wide range of colors. Useful applications such as flat-panel displays have been contemplated. Representative of earlier organic EL devices are Gurnee et al. U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. Typical organic emitting materials were formed of a conjugated organic host material and a conjugated organic activating agent having condensed benzene rings. The organic emitting material was present as a single layer medium having a thickness much above 1 micrometer. Thus, this organic EL medium was highly resistive and the EL device required an extremely high voltage (&gt;100 volts) to operate.
The most recent discoveries in the art of organic EL device construction have resulted in devices having the organic EL medium consisting of extremely thin layers (&lt;1.0 micrometer in combined thickness) separating the anode and cathode. The thin organic EL medium offers reduced resistance, permitting higher current densities for a given level of electrical bias voltage. In a basic two-layer EL device structure, one organic layer is specifically chosen to inject and transport holes and the other organic layer is specifically chosen to inject and transport electrons. The interface between the two layers provides an efficient site for the recombination of the injected hole-electron pair and resultant electroluminescence. Examples are provided by U.S. Pat. Nos. 4,356,429; 4,539,507; 4,720,432; 4,885,211; 4,950,950; 5,047,687; 5,059,861; 5,061,569; 5,073,446; 5,141,671; 5,150,006; and 5,151,629.
The simple structure can be modified to a three-layer structure, in which an additional electroluminescent layer is introduced between the hole and electron transporting layers to function primarily as the site for hole-electron recombination and thus electroluminescence. In this respect, the functions of the individual organic layers are distinct and can therefore be optimized independently. Thus, the electroluminescent or recombination layer can be chosen to have a desirable EL color as well as a high luminance efficiency. Likewise, the electron and hole transport layers can be optimized primarily for the carrier transport property.
The organic EL devices can be viewed as a diode which is forward biased when the anode is at a higher potential than the cathode. The anode and cathode of the organic EL device can each take any convenient conventional form, such as any of the various forms disclosed by Tang et al. U.S. Pat. No. 4,885,211. Operating voltage can be substantially reduced when using a low-work function cathode and a high-work function anode. The preferred cathodes are those constructed of a combination of a metal having a work function less than 4.0 eV and one other metal, preferably a metal having a work function greater than 4.0 eV. The Mg:Ag of Tang et al. U.S. Pat. No. 4,885,211 constitute one preferred cathode construction. The Al:Mg cathodes of VanSlyke et al. U.S. Pat. No. 5,059,062 are another preferred cathode construction. The use of a LiF/Al bilayer to enhanced electron injection in organic EL devices has been disclosed by Hung et al. in U.S. Pat. No. 5,776,622.
Conventional anodes are formed of a conductive and transparent oxide. Indium tin oxide has been widely used as the anode contact because of its transparency, good conductivity, and high work function. However, a device grown on a bare ITO surface generally shows insufficient hole injection and poor operational stability. The mitigation of these problems has involved introducing an intermediate layer between the ITO and the organic medium. Yang et al. reported a polymer EL device with improved charge carrier injection by using a polyaniline layer between the ITO and active electroluminescent layer. See "Enhanced performance of polymer light-emitting diodes using high-surface area polyaniline network electrodes" by Y. Yang, E. Westerweele, C. Zhang, P. Smith, and A. J. Heeger, Journal of Applied Physics Letters, Vol. 77, 694 (1995). VanSlyke et al. demonstrated a highly stable organic device formed by using a CuPc layer between the ITO and the hole-transporting layer. See "Organic electroluminescent devices with improved stability" by S. A. VanSlyke, C. H. Chen, and C. W. Tang, Applied Physics Letters, Vol. 69, 2160 (1996). We also found that the indium-tin-oxide (ITO) anode contact to an organic EL device can be significantly improved via oxygen plasma treatment without introducing a CuPc layer. The resulting device can be operated at low voltages, and exhibits good stability. The observation is consistent with the finding by Wu et al. in polymer light-emitting diodes. See "Surface modification of indium tin oxide by plasma treatment: an effective method to improve the efficiency, brightness, and reliability of organic light emitting devices" by C. C. Wu, C. I. Wu, J. C. Sturm, and A. Kahn, Applied Physics Letters, Vol. 70, 1348 (1997). Notwithstanding the foregoing, there is still a need to further enhance hole injection and improve operational stability of electroluminescent devices.